For many years decorative laminates have been used as a surfacing material in residential and commercial structures wherein aesthetic effects are desired in combination with functional behavior such as wear, heat and stain resistance. Typical applications of said laminates are surfacing for walls, partitions, table tops, counter tops, furniture, doors and the like. Such decorative laminates generally are produced from a plurality of resin impregnated core sheets usually composed of kraft paper which has been impregnated with a thermosetting resin and, more particularly, with a thermosetting water-soluble or water-insoluble phenolic resin. When the kraft paper has been impregnated with the thermosetting resin, the sheets are dried and cut to the appropriate size. Thereupon, a plurality of these resin impregnated sheets are stacked in a superimposed relationship. The number of plies or sheets in the stack depends on the ultimate intended use of the laminate. For most purposes, the number of plies of these core sheets will total about six to nine but can total as many as 12-15.
There is placed on the stack of core sheets a decorative sheet which is generally a sheet of alpha-cellulose paper bearing a printed design or a light color and impregnated with a noble thermosetting resin which is not subject to significant darkening upon the application of heat. Suitable resins for the decorative sheets are the aminotriazine resins and more particularly the melamine-formaldehyde resins, the benzoguanamine-formaldehyde resins and the unsaturated polyester resins and the like. It is generally desirable when making decorative laminates to make use of a protective overlay sheet which is placed atop and is similar to the decorative sheet but is generally devoid of design and in the final laminate is transparent. The stack of impregnated sheets is inserted into a laminating press and is heat and pressure consolidated to a unitary structure. During the consolidation step, the thermosetting resins are converted to the thermoset state thereby providing an extremely hard, attractive and permanent surfacing material. For obvious economic reasons, it is common practice to consolidate a plurality of these individual laminating assemblies into one large assembly, or press pack, said stacks being separated from one another by a release sheet, and then to laminate this pack by heat and pressure application.
In building such a pack, an individual assembly is placed with its decorative overlayment surface adjacent to a highly polished stainless steel press plate. The function of the press plate is twofold. First, it provides a smooth, defect-free surface to one side of the laminate. Second, it serves to separate pairs of back-to-back assemblies, thus permitting a plurality of these assemblies to be consolidated into laminates in one operation, usually in back-to-back relationship.
The press pack is then pressed or molded. This is accomplished by placing the pack between the platens of a hydraulic press. The press usually has multiple openings so that up to 22 or more packs may be pressed at once. Such an approach produces a very smooth surface laminate with a glossy finish. Other techniques are used to produce laminates with a smooth surface finish but with a satin or brushed textured effect.
More recently, the dictates of fashion have demanded textured laminates which have a three dimensional surface configuration. Earlier investigators have used either embossed, machined or etched three dimensional metal press plates in making such decorative laminates, directly from these plates. While successful, such procedures have proven exceedingly expensive because of the high cost of preparation and maintenance of each plate. Other investigators proposed making the plates out of other materials having the desired configuration but these were not entirely satisfactory because of the failure, in one way or another, of these materials during the high temperatures and pressures used in laminating.
U.S. Pat. No. 3,303,081 to Michaelson et al. solves some, but by no means all, of the problems related to making dimensional decorative laminates by teaching a method of making a negative surface laminate "master" which is to be then used to prepare decorative laminates of conventional construction. In all the disclosed examples, a phenolic resin impregnated kraft paper core is used in production of the master. A later U.S. patent to Michaelson states that the masters made according to U.S. Pat. No. 3,303,081 have a limited life of forty pressings, after which they delaminate and become ineffective for further molding. This later U.S. Pat. No. 3,311,520, solves the delamination problem of the former by incorporating into the master a dimensionally stable metal core. While the metal core masters are more useful in that they can be used many more times without delamination to produce decorative laminates or additional masters, they are themselves quite heavy and expensive and tend to present problems with regard to the adhesion of the metal to the Kraft paper which is present on its planar surfaces and constitutes the remainder of the core of the master.